Nearly 1 million children in the United States are potentially misdiagnosed with attention deficit hyperactivity disorder simply because they are the youngest – and most immature – in their kindergarten class, according to new research by , Todd Elder, a Michigan State University economist.

These children are significantly more likely than their older classmates to be prescribed behavior-modifying stimulants such as Ritalin, said Todd Elder, whose study will appear in a forthcoming issue of the Journal of Health Economics (closed science, unfortunately). Michigan State should stop funding closed journals with free content – other schools have decided to put science first, before supporting a few outdated business models of select journals.

Such inappropriate treatment is particularly worrisome because of the unknown impacts of long-term stimulant use on children’s health, Elder said. It also wastes an estimated $320 million-$500 million a year on unnecessary medication – some $80 million-$90 million of it paid by Medicaid, he said.

ADHD is the most commonly diagnosed behavioral disorder for kids in the United States, with at least 4.5 million diagnoses among children under age 18, according to the Centers for Disease Control and Prevention.

The youngest kindergartners were 60 percent more likely to be diagnosed with ADHD than the oldest children in the same grade. Similarly, when that group of classmates reached the fifth and eighth grades, the youngest were more than twice as likely to be prescribed stimulants.

Overall, the study found that about 20 percent – or 900,000 – of the 4.5 million children currently identified as having ADHD likely have been misdiagnosed.